This article presents a theoretical analysis on the thermal buckling behavior of sandwich panels with truss cores under fully clamped boundary conditions, subjected to uniform temperature rise. The Reissner model is developed by ignoring the flexural rigidity of the core and considering the shear stiffness of the sandwich panel is only contributed by truss cores. By using double Fourier expansions to the virtual deformation mode, the critical temperature of sandwich panels is obtained. Theoretically predicted critical temperatures are in good agreement with those from FEM. The effect of boundary conditions and structure parameters of the sandwich panel are also discussed.